The present invention relates to a flow meter for indicating fluid flow. It is particularly, but not exclusively, related to indicating or measuring the flow of oil and water mixtures in production tubing of a well.
A well used to extract hydrocarbons from an underground reservoir typically comprises a borehole drilled down to the underground reservoir and lined with a steel casing. The casing is inserted into the borehole and cemented into place. Production tubing, which is used to convey hydrocarbons from the reservoir to an upper region, or mouth, of the borehole, extends from the mouth of the borehole to the reservoir. The reservoir is typically located near to the bottom of the borehole. An elongate annular space thus defined between the casing and the tubing, referred to as the annulus, is usually sealed at points along its length by devices known as packers.
In operation of the well, hydrocarbons enter the production tubing either through its open lower end or through a choke device located at a position along its length between two packers. It has been proposed to make measurements of hydrocarbons"" flow rate, temperature and pressure in the borehole and use the measurements to control the choke devices.
Since hydrocarbons may enter the production tubing at different locations the need arises to measure hydrocarbons"" flow rate with a flow meter at different locations within the production tubing. A problem found with certain conventional flow meters is that they reduce the effective diameter of the bore of the production tubing and must be removed to allow the passage of tools to upstream tubing. The term effective diameter here means the smallest unobstructed diameter and can be hypothetically equated to the largest diameter circular tube which could be passed lengthwise through the insert I without obstruction. Removal of such flow meters is costly, time consuming and risky. Another problem is that at low flow rates, certain conventional flow meters can give less accurate or lower resolution measurements than is desired.
In accordance with the present invention there is provided a flow meter for indicating fluid flow in a duct, comprising a pocket open at its upstream end in the duct, detection means for detecting a pressure differential between a first position inside the pocket and a second position outside the pocket and means for deriving from the pressure differential an indication of the fluid flow within the duct.
At low flow rates, a flow meter according to the invention can provide more accurate or higher resolution flow indication or measurement using the same or even inferior accuracy pressure transducers than has been possible hitherto. An advantage of the invention is that it need not reduce the effective diameter of a duct in which it is installed.
The duct may be a conduit, a pipe or some other fluid path.
The pocket may be closed at its downstream end. Alternatively it may be open at its downstream end and be provided by at least one hole through which fluid is able to flow.
The pocket may extend continuously around the internal perimeter of the flow meter. Advantageously, this provides a relatively simple configuration. Alternatively the pocket extends around a segment of the internal perimeter of the flow meter. Such a configuration may be of particular use if the dimensions of the insert are limited by an external factor.
The flow meter comprises a central bore region. This may have a bore having a non-circular cross-section. Preferably it is oval. Use of such a cross-section enables the cross-sectional area of the bore of the central bore region to be increased relative to that available if a bore having a circular cross-section is used. In this event the effective diameter may be limited by features on the outside surface of the insert. A non-circular bore may be of particular advantage in boreholes where the outer dimensions of the flow meter are limited by the inner dimensions of, for example, a borehole casing.
Preferably, the segment corresponds to an apex of an oval bore. Here, the effective diameter of the bore of the flow meter may be greater than or equal to the bore of an inlet or an outlet of the flow meter for the whole of its length. This may be advantageous in providing a flow meter which permits the passage of tools and instrumentation through production tubing in which it is installed.
Preferably the flow meter has a first conduit allowing communication of pressure information from a first position in its bore to a measuring position at its outer surface.
It may also have a second conduit allowing communication of pressure information from a second position in its bore to a measuring position at its outer surface. This allows pressure measurement transducers and any associated electronics to be located outside the duct, away from the fluid flow, to increase its lifetime, reliability, or both. This is advantageous if the fluid flow provides a harsh environment. Preferably, the first conduit extends first substantially radially and then substantially longitudinally. This feature may allow the flow meter to occupy minimal space radially from the central axis of the duct. This is of particular significance where the insert is to be used within a hydrocarbon production borehole. It is also of significance in a number of narrow bore holes for other purposes.